The present invention relates in general to signal detection and analysis systems and components therefor, and is particularly directed to a new and improved acoustic signal detector, such as may be employed in a hydrophone and the like, having an acoustic stimulus sensitivity characteristic that is controllably variable by adjusting the gauge length of optical waveguide forming the sensor.
The accurate detection and measurement of signals emanating from one or more remote or local sources, such as but not limited to acoustic energy sources, are fundamental requirements of a variety of industrial, military, and scientific systems. Because characteristics of the signals being measured not only typically vary among different applications, but may manifest substantial changes for a given application, the system designer is typically faced with having to trade off between sensitivity and dynamic range, when choosing a transducer/sensor.
Attempts to solve this problem have included coupling the output of the sensor to a variable gain amplifier, and adjusting the amplifier gain in accordance with the expected characteristics of the signal being monitored. An obvious deficiency to this approach is the fact that controlling the operation of downstream electronics will not vary the sensitivity of the upstream sensor. In addition, this scheme is noisy at higher gains and the sensitivity range is narrower. Another technique has been to multiplex the outputs of a plurality of different sensitivity transducers. Not only does this increase hardware, signal processing complexity and cost, but compromises the required location of the sensor.
In accordance with the present invention, these shortcomings of conventional fixed and pseudo variable sensitivity (acoustic) sensor architectures are successfully addressed by an acoustic signal detector having a variable sensitivity characteristic, in particular a variable gauge length, that is controllably and dynamically modified by adjusting the location of a light reflection interface within a section of optical waveguide to which the acoustic stimulus to be sensed is applied. By changing the position of the light reflection interface to increase the gauge length, the distance over which the refractive index of the waveguide is changed as a result the acoustic stimulus is increased, making the sensor more sensitive to small amplitude signals. By decreasing the distance over which the refractive index of the waveguide is affected by the acoustic stimulus, the gauge length and sensitivity of the sensor is decreased, so as to tune the sensor""s sensitivity to large amplitude signals.
In a preferred embodiment, the variable gauge length sensor of the invention is configured as an interferometer-based architecture. A light beam such that generated by a laser is applied via an optical waveguide coupler to each of an xe2x80x98acoustic signal detectionxe2x80x99 section of optical waveguide and a xe2x80x98referencexe2x80x99 section of optical waveguide. The coupler also has an output port coupled to a photodetector.
The xe2x80x98acoustic signal detectionxe2x80x99 section of optical waveguide is coupled to an acoustic energy transmission element through which an input acoustic stimulus to be measured/sensed is impressed upon the signal waveguide section, and thereby modifies the index of refraction of the optical waveguide material, modulating the light passing through the waveguide in accordance with the acoustic signal. The gauge length of the xe2x80x98acoustic signal detectionxe2x80x99 section of optical waveguide is defined by the displacement of a reflection interface from the waveguide coupler. The greater the displacement, the longer the two-way xe2x80x98signalxe2x80x99 travel path of the light beam through the acoustic stimulus-receiving optical waveguide section from the coupler to the reflection interface and back. Since the index of refraction of the optical waveguide section is modified by the acoustic stimulus, the signal beam will undergo a phase delay that is dependent upon the amplitude of the acoustic signal being measured and the gauge length through the signal waveguide section.
The xe2x80x98referencexe2x80x99 optical waveguide section also contains a reflection interface, the position of which is ganged with the reflection interface of the signal optical waveguide section. This results in a two-way travel path of the xe2x80x98referencexe2x80x99 light beam, through the reference optical waveguide section from the coupler to its reflection interface and back, being the same beam travel distance as the signal beam in the signal optical waveguide section. The two xe2x80x98signalxe2x80x99 path and xe2x80x98referencexe2x80x99 path beams are respectively reflected back into the coupler by their reflection interfaces and are combined at the output port of the coupler and applied to the photo detector. The index of refraction of the material of the signal optical waveguide section is modified by the acoustic stimulus is the xe2x80x98signal pathxe2x80x99. This xe2x80x98signalxe2x80x99 path light beam is combined out of phase with xe2x80x98referencexe2x80x99 light beam at the detector.
Non-limiting examples of mechanisms for controllably varying the locations of the respective reflection interfaces along the signal and reference waveguide sections include physically displaceable mirrors and electro-thermally driven strips. The mirrors are controllably positionable in the signal and reference light beam travel paths through associated cascaded sections of optical waveguide. Through electromagnetic solenoid drivers, selected ganged pairs of mirrors may be controllably positioned within the signal and reference beam travel paths, so as to incrementally or stepwise change the gauge length of the sensor. Similarly, supplying electrical current to selected ganged pairs of the thermal strips induces reflection interfaces in the beam travel paths through signal and reference waveguide sections and thereby incrementally or stepwise changes the gauge length of the acoustic sensor.